Capsaicin is the chemical compound responsible for the burning heat of chili peppers.
Capsaicin activates TRPV1 receptors on pain-sensing nerve cells. These receptors normally respond to high temperatures and physical irritation, so the brain interprets capsaicin exposure as heat even though the pepper is not actually hot. The resulting sensation can include burning, tingling, itching, and stinging.
Chili peppers produce capsaicin and related compounds called capsaicinoids as secondary metabolites. The highest concentration is usually found in the white placental tissue and inner membranes where the seeds are attached—not in the seeds themselves. This chemistry may help deter mammals and fungal pathogens while allowing birds, which are largely insensitive to capsaicin, to disperse the seeds.
People often assume that drinking water is the best way to cool chili heat, but capsaicin is hydrophobic and does not dissolve readily in water. Milk can work better because its casein proteins help lift the compound away from receptors. Pepper heat is commonly rated using the Scoville scale, which measures pungency rather than literal temperature.